Microstructure

ABSTRACT

A microstructure with regularly arranged micropores is obtained by anodizing an aluminum layer having a Vickers hardness Hv of up to 20. The microstructure has a well-ordered region of large surface area and can be manufactured at low cost.

BACKGROUND OF THE INVENTION

The present invention relates to a microstructure including an aluminum layer having on a surface thereof an anodized layer containing a plurality of micropores and a method of producing the same.

In the technical field of metal and semiconductor thin films, wires and dots, it is known that the movement of free electrons becomes confined at sizes smaller than some characteristic length, as a result of which singular electrical, optical and chemical phenomena become observable. Such phenomena are called “quantum mechanical size effects” or simply “quantum size effects.” Functional materials which employ such singular phenomena are under active research and development. Specifically, materials having structures smaller than several hundred nanometers in size, typically called microstructures or nanostructures, are the subject of current efforts in material development.

Methods for manufacturing such microstructures include processes in which a nanostructure is directly manufactured by semiconductor fabrication technology, including micropatterning technology such as photolithography, electron beam lithography, or x-ray lithography.

Of particular note is the considerable amount of research being conducted today on processes for manufacturing nanostructures having an ordered microstructure.

One method of forming an ordered structure in a self-regulating manner is illustrated by an anodized alumina layer (anodized layer) obtained by subjecting aluminum to anodizing treatment in an electrolyte solution. It is known that a plurality of micropores having diameters of about several nanometers to about several hundreds of nanometers are formed in a regular arrangement within the anodized layer. It is also known that when a completely ordered arrangement is obtained by the self-ordering treatment of this anodized layer, hexagonal columnar cells will be theoretically formed, each cell having a base in the shape of a regular hexagon centered on a micropore, and that the lines connecting neighboring micropores will form equilateral triangles.

For example, H. Masuda et al. (Jpn. J. Appl. Phys. Vol. 37, Part 2, No. 11A, pp. L1340-1342 (Nov. 1, 1998), FIG. 2) describes an anodized layer having micropores. In another related publication (Hyomen Gijutsu Binran [Handbook of Surface Technology], edited by The Surface Finishing Society of Japan (Nikkan Kogyo Shimbun Co., Ltd., 1998), pp. 490-553), it is described that micropores are naturally formed in an anodized layer as oxidation proceeds. Moreover, H. Masuda (“Highly ordered metal nanohole array based on anodized alumina”, Kotai Butsuri [Solid State Physics], Vol. 31, No. 5, pp. 493-499 (1996)) has proposed the formation of a gold dot array on a silicon substrate using a porous anodized layer as the mask.

A plurality of micropores take on a honeycomb-like structure in which the pores are formed parallel in a direction substantially vertical to the substrate surface, and at substantially equal intervals. This point is deemed to be the most distinctive characteristic of anodized layers in terms of material. Another remarkable feature of anodized layers, thought to be absent in other materials, is the ability to relatively freely control the pore diameter, pore spacing and pore depth (see Masuda, 1996).

Known examples of applications for anodized layers include various types of devices, such as nanodevices (e.g., ultra-microfilters), magnetic devices (e.g., high-density magnetic recording media), and light-emitting devices. For example, JP 2000-31462 A mentions a number of applications, including magnetic devices in which the micropores are filled with the magnetic metal cobalt or nickel, light-emitting devices in which the micropores are filled with the luminescent material ZnO, and biosensors in which the micropores are filled with enzymes/antibodies.

In addition, in the field of biosensing, JP 2003-268592 A describes an example in which a structure obtained by filling the interior of micropores in an anodized layer with a metal is used as a sample holder for Raman spectroscopy.

In such practical applications, the surface area of the region over which an ordered array has been preserved must be increased. If the period over which the regularity of the anodized layer can be preserved is too short, it may not be possible to elicit the performance required of the microstructure.

Commonly assigned JP 2004-107770 A discloses that the regularity of the pore pattern can be enhanced by anodizing a surface layer of high chemical purity.

SUMMARY OF THE INVENTION

However, in anodized layers obtained by conventional methods, even when the chemical purity of the material is increased, the period over which the regularity can be preserved in the anodized layer is short, making it difficult to achieve a mean domain size larger than about 2 μm. As used herein, “domain” refers to a region over which orderliness, or regularity, is preserved. FIG. 1 is a schematic diagram showing the surface of an aluminum substrate having an anodized layer in which micropores 5 and cells 10 are present in a regular arrangement. This regularity, as indicated by two chain lines, can be seen to extend over two differing domains: a first domain 11 and a second domain 12. Because the domains, which are discrete regions having a regularly arranged structure, change due to slight fluctuations in the conditions during anodizing treatment and due also to transitions and lattice defects in the aluminum solid, a plurality of domains are formed on the aluminum surface. In the anodization step, the domains have a mean size of about 0.5 to 1 μm when an oxalic acid electrolyte solution is used, 0.5 μm or less when a sulfuric acid electrolyte solution is used, and at most 2 μm even with the use of a phosphoric acid electrolyte solution.

In the course of investigating whether other elements in the form of impurities or the like are detectable in the region of such domains, the inventors have found that the regularity of the domains can be enhanced by setting the hardness of the aluminum material prior to anodization at or below a specific value.

The inventors have also found that it is necessary to control the hardness of the aluminum plate in order to maximize the size of these discrete, well-ordered regions (domains) on an aluminum microstructure bearing an anodized layer so that the microstructure obtained has domains of a large size, and ultimately the domains on the anodized layer can be enlarged.

It is therefore one object of the invention is to provide a large-surface-area, low-cost microstructure composed of an anodized layer-bearing aluminum plate which contains regularly arranged pores and in which the well-ordered region has a large surface area.

Another object of the invention is to provide a method of producing such microstructure.

Accordingly, the invention provides the following aspects (1) to (4).

-   (1) A microstructure comprising regularly arranged micropores which     is obtained by anodizing an aluminum plate having a Vickers hardness     Hv of up to 20. -   (2) The microstructure of (1) above which is obtained by     heat-treating the aluminum plate at least once at a temperature of     at least 200° C. for at least one hour before the aluminum plate is     anodized. -   (3) The microstructure of (1) or (2) above, wherein the regularity     of the micropores, expressed as the mean domain size, is at least 2     μm. -   (4) A method of producing a microstructure comprising the steps of:

heat-treating an aluminum plate at least once at a temperature of at least 200° C. for at least one hour; and

anodizing the heat-treated aluminum plate.

As noted above, the invention provides microstructures in which the regions where an ordered structure is preserved (domains) are of a large size. The microstructures of the invention have regular arrangement preserving regions (domains) of larger surface area, which helps improve yield when such microstructures are used to obtain devices such as those mentioned above.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a schematic diagram showing the micropore regularity and domains on an anodized surface; and

FIG. 2 is a schematic diagram showing the pores on an anodized surface and illustrating how the domain size is measured.

DETAILED DESCRIPTION OF THE INVENTION

<Aluminum Layer>

The inventive structure is a microstructure composed entirely or in part of an aluminum layer having on the surface thereof a micropore-bearing anodized layer.

The aluminum layer having the anodized surface layer which is used in the invention can be obtained by anodizing the surface of the aluminum layer.

Any member may be used as long as the member has the aluminum layer. Illustrative examples include aluminum substrates such as substrates made of low-purity aluminum (e.g., recycled material) onto which high-purity aluminum has been vapor-deposited; silicon wafers or substrates made of quartz or glass, the surface of which has been covered with high-purity aluminum by a suitable technique such as sputtering, vapor deposition, chemical vapor deposition, electrodeposition, chemical plating or electroplating; and metal or resin substrates on which an aluminum layer has been laminated over an intervening layer of adhesive. Alternatively, the substrate and the aluminum layer may together constitute an aluminum plate.

In cases where pits serving as the starting points of the main anodizing treatment are to be formed by the self-ordering method as described later, the member which has the aluminum layer must have a certain degree of thickness. For this reason, an aluminum plate is preferred. Hence, in the description that follows, an aluminum plate is used as an example of the aluminum layer.

<Aluminum Plate (Rolled Aluminum)>

In the microstructure of the invention, the aluminum plate may be produced from a known aluminum material. The aluminum plate used in the invention is made of a dimensionally stable metal composed primarily of aluminum; that is, aluminum or an aluminum alloy. Aside from a plate of pure aluminum, alloy plate composed primarily of aluminum and containing trace amounts of other elements can also be used.

In the present specification, the various above-mentioned substrates made of aluminum or aluminum alloy are referred to generically as “aluminum plate.” Other elements which may be present in the aluminum alloy include silicon, iron, manganese, copper, magnesium, chromium, zinc, bismuth, nickel and titanium. The aluminum purity is preferably at least 99.5 wt %, and the silicon content is most preferably not more than 0.01 wt %. The reason is that silicon is an element which readily precipitates; depending on factors such as the heat treatment conditions, silicon may give rise to grain boundaries, making the domain size smaller. The aluminum purity is more preferably at least 99.9 wt %, and even more preferably at least 99.99 wt %. At an aluminum purity in the above range, the regularity of the micropore arrangement increases.

The aluminum alloy may be formed into a plate by the following method, for example. First, an aluminum alloy melt that has been adjusted to a given alloying ingredient content is subjected to cleaning treatment by an ordinary method, then is cast. Cleaning treatment, which is carried out to remove hydrogen and other unwanted gases from the melt, typically involves flux treatment; degassing treatment using argon gas, chlorine gas or the like; filtering treatment using, for example, what is referred to as a rigid media filter (e.g., ceramic tube filter, ceramic foam filter), a filter that employs a filter medium such as alumina flakes or alumina balls, or a glass cloth filter; or a combination of degassing treatment and filtering treatment.

Cleaning treatment is preferably carried out to prevent defects due to foreign matter such as nonmetallic inclusions and oxides in the melt, and defects due to dissolved gases in the melt. The filtration of melts is described in, for example, JP 6-57432 A, JP 3-162530 A, JP 5-140659 A, JP 4-231425 A, JP 4-276031 A, JP 5-311261 A, and JP 6-136466 A. The degassing of melts is described in, for example, JP 5-51659 A and JP 5-49148 U. The present applicant discloses related art concerning the degassing of melts in JP 7-40017 A.

Next, the melt that has been subjected to cleaning treatment as described above is cast. Casting processes include those which use a stationary mold, such as direct chill casting, and those which use a moving mold, such as continuous casting.

In direct chill casting, the melt is solidified at a cooling speed of 0.5 to 30° C./s. At less than 1° C./s, many coarse intermetallic compounds may be formed. When direct chill casting is carried out, an ingot having a thickness of 300 to 800 mm can be obtained. If necessary, this ingot is scalped by a conventional method, generally removing 1 to 30 mm, and preferably 1 to 10 mm, of material from the surface. The ingot may also be optionally soaked, either before or after scalping. In cases where soaking is carried out, the ingot is heat-treated at 450 to 620° C. for 1 to 48 hours to prevent the coarsening of intermetallic compounds. The effects of soaking treatment may be inadequate if heat treatment time is shorter than one hour.

The ingot is then hot-rolled and cold-rolled, giving a rolled aluminum plate. A temperature of 350 to 500° C. at the start of hot rolling is appropriate. Intermediate annealing may be carried out before or after hot rolling, or even during hot rolling. The intermediate annealing conditions may consist of 2 to 20 hours of heating at 280 to 600° C., and preferably 2 to 10 hours of heating at 350 to 500° C., in a batch-type annealing furnace, or of heating for up to 6 minutes at 400 to 600° C., and preferably up to 2 minutes at 450 to 550° C., in a continuous annealing furnace. Using a continuous annealing furnace to heat the rolled plate at a temperature rise rate of 10 to 200° C./s enables a finer crystal structure to be achieved.

The aluminum plate that has been finished by the above step to a given thickness of, say, 0.1 to 0.5 mm may then be passed through a leveling machine such as a roller leveler or a tension leveler to improve the flatness. The flatness may be improved in this way after the continuous aluminum plate has been cut into discrete sheets. However, to enhance productivity, it is preferable to carry out such flattening with the rolled aluminum in the state of a continuous coil. The plate may also be passed through a slitter line to cut it to a predetermined width. A thin film of oil may be provided on the aluminum plate to prevent scuffing due to rubbing between adjoining aluminum plates. Suitable use may be made of either a volatile or non-volatile oil film, as needed.

Continuous casting processes that are industrially carried out include processes which use cooling rolls, such as the twin roll process (HIiJnter process) and the 3C process; and processes which use a cooling belt or a cooling block, such as the twin belt process (Hazelett process) and the Alusuisse Caster II process. When a continuous casting process is used, the melt is solidified at a cooling rate of 100 to 1,000° C./s. Continuous casting processes generally have a faster cooling rate than direct chill casting processes, and so are characterized by the ability to achieve a higher solid solubility of alloying ingredients in the aluminum matrix. Technology relating to continuous casting processes that has been disclosed by the present applicant is described in, for example, JP 3-79798 A, JP 5-201166 A, JP 5-156414 A, JP 6-262203 A, JP 6-122949 A, JP 6-210406 A and JP 6-26308 A.

When continuous casting is carried out, such as by a process involving the use of cooling rolls (e.g., the Hunter process), the melt can be directly and continuously cast as a plate having a thickness of 1 to 10 mm, thus making it possible to omit the hot rolling step. Moreover, when use is made of a process that employs cooling belts (e.g., the Hazelett process), a plate having a thickness of 10 to 50 mm can be cast. Generally, by positioning a hot-rolling roll immediately downstream of the caster, the cast plate can then be successively rolled without interruption, making it possible to obtain a continuously cast and rolled plate having a thickness of 1 to 10 mm.

These continuously cast and rolled plates are then passed through such steps as cold rolling, intermediate annealing, flattening and slitting in the same way as described above for direct chill casting, and thereby finished to a plate thickness of typically 0.1 to 0.5 mm. Technology disclosed by the present applicant concerning the intermediate annealing conditions and cold rolling conditions in a continuous casting process is described in, for example, JP 6-220593 A, JP 6-210308 A, JP 7-54111 A and JP 8-92709 A.

Because the crystal structure at the surface of the aluminum plate may give rise to a poor surface quality when chemical graining treatment or electrochemical graining treatment is carried out, it is preferable for the crystal structure to not be too coarse. The crystal structure at the surface of the aluminum plate has a width of preferably 200 μm or less, more preferably 100 μm or less, and most preferably 50 μm or less. Moreover, the crystal structure has a length of preferably 5,000 μm or less, more preferably 1,000 μm or less, and most preferably 500 μm or less. Related technology disclosed by the present applicant is described in, for example, JP 6-218495 A, JP 7-39906 A and JP 7-124609 A.

A proportional relationship exists between the Vickers hardness of the aluminum base metal and the tensile strength of the aluminum. In the case of a rolled material, the hardness can be largely controlled by adjusting the tensile strength. This tensile strength can be regulated by means of certain processes used to obtain the aluminum plate, such as cold working, solution treatment, age hardening treatment and annealing. Strength changes from annealing are mentioned in Aluminum Handbook (published by the Japan Aluminum Association) and other sources.

The aluminum alloy composition also influences strength. 7000 series alloys containing zinc and magnesium have a strength comparable to carbon steel, but high-purity 1000 series alloys exhibit a strength only about one-tenth as large.

In this embodiment of the invention, the aluminum material is not limited to a rolled material, and may instead be an aluminum layer obtained by a process such as hot-dip coating or vapor deposition. Annealing treatment releases residual stress in the material, and may thus lower the hardness.

<Heat Treatment>

In the practice of the invention, when the above-described aluminum plate, prior to anodization, is subjected at least once to a heat treatment step in which it is held at a temperature of at least 200° C. for at least 1 hour, the grain boundaries present in the aluminum layer decrease, thus making it possible to obtain a microstructure having an anodized layer in which the domain size is large. Heat treatment is carried out preferably at 200° C. for 2 or 3 hours, more preferably at 250° C. for 1, 2 or 3 hours, and even more preferably at 300 to 400° C. for 1, 2 or 3 hours. Alternatively, heat treatment may be carried out twice at 250° C. for 2 hours each time, or twice at 300° C. for 2 hours each time. When the regions in which the pore arrangement is highly regular increase in size as a result of heat treatment, sensing light reaches the detector without irregular reflection, increasing the precision of measurement. Following heat treatment, the aluminum substrate may be cooled by direct immersion in water or the like, but is preferably allowed to cool in air.

When heat treatment is carried out under preferred conditions in the above-indicated range, the mean domain size following anodization can easily be made 2 μm or larger.

The aluminum substrate is preferably subjected to degreasing and mirror-like finishing prior to anodizing treatment.

<Degreasing>

Degreasing is carried out with a suitable substance such as an acid, alkali or organic solvent so as to dissolve and remove organic substances, including dust, grease and resins, adhering to the aluminum surface, and thereby prevent defects caused by organic substances from arising in each of the subsequent treatments.

Known degreasers can be used in degreasing treatment. For example, degreasing can be carried out using any of various commercially available degreasers by the prescribed method.

Preferred methods include the following: a method in which an organic solvent such as an alcohol (e.g., methanol), a ketone, benzine or a volatile oil is brought into contact with the aluminum surface at ambient temperature (organic solvent method); a method in which a liquid containing a surfactant such as soap or a neutral detergent is brought into contact with the aluminum surface at a temperature of from ambient temperature to 80° C., after which the surface is rinsed with water (surfactant method); a method in which an aqueous sulfuric acid solution having a concentration of 10 to 200 g/L is brought into contact with the aluminum surface at a temperature of from ambient temperature to 70° C. for a period of 30 to 80 seconds, following which the surface is rinsed with water; a method in which an aqueous solution of sodium hydroxide having a concentration of 5 to 20 g/L is brought into contact with the aluminum surface at ambient temperature for about 30 seconds while electrolysis is carried out by passing a direct current through the aluminum surface as the cathode at a current density of 1 to 10 A/dm², following which the surface is brought into contact with an aqueous solution of nitric acid having a concentration of 100 to 500 g/L and thereby neutralized; a method in which the aluminum surface is brought into contact with any of various known anodizing electrolyte solutions at ambient temperature while electrolysis is carried out by passing a direct current at a current density of 1 to 10 A/dm² or an alternating current through the aluminum surface as the cathode; a method in which an aqueous alkali solution having a concentration of 10 to 200 g/L is brought into contact with the aluminum surface at 40 to 50° C. for 15 to 60 seconds, following which the surface is brought into contact with an aqueous nitric acid solution having a concentration of 100 to 500 g/L and thereby neutralized; a method in which an emulsion prepared by mixing a surfactant, water or the like into an oil such as gas oil or kerosene is brought into contact with the aluminum surface at a temperature of from ambient temperature to 50° C., following which the surface is rinsed with water (emulsion degreasing method); and a method in which a mixed solution of, for example, sodium carbonate, phosphates and surfactant is brought into contact with the aluminum surface at a temperature of ambient temperature to 50° C. for 30 to 180 seconds, following which the surface is rinsed with water (phosphate method).

The method used for degreasing is preferably one which can remove grease from the aluminum surface but causes substantially no aluminum dissolution. Hence, an organic solvent method, surfactant method, emulsion degreasing method or phosphate method is preferred.

<Mirror-Like Finishing>

Mirror-like finishing is carried out to eliminate surface asperities on the aluminum substrate and improve the uniformity and reproducibility of grain-forming treatment by a process such as electrodeposition. Examples of surface asperities on the aluminum substrate include rolling streaks formed during rolling when the aluminum substrate has been produced by a process that includes rolling. The purpose for carrying out mirror-like finishing is to eliminate asperities on the aluminum surface and enhance uniformity at the time of pre-anodizing treatment.

In the practice of the invention, mirror-like finishing is not subject to any particular limitation, and can be carried out using any suitable method known in the art. Examples of suitable methods include mechanical polishing, chemical polishing, and electrolytic polishing.

Illustrative examples of suitable mechanical polishing methods include polishing with various commercial abrasive cloths, and methods that combine the use of various commercial abrasives (e.g., diamond, alumina) with buffing. More specifically, preferred methods involving the use of abrasives include a method that is carried out while varying over time the abrasive used from coarser particles to finer particles. In such a case, the final abrasive used is preferably one having a grit size of 1500. In this way, a glossiness of at least 50% (in the case of rolled aluminum, at least 50% in both the rolling direction and the transverse direction) can be achieved.

Examples of chemical polishing methods include the various methods mentioned in the 6^(th) edition of Aluminum Handbook (Japan Aluminum Association, 2001), pp. 164-165.

Preferred examples include the phosphoric acid/nitric acid method, Alupol I, Alupol V, Alcoa R5, the H₃PO₄—CH₃COOH—Cu method and the H₃PO₄—HNO₃—CH₃COOH method. Of these, the phosphoric acid/nitric acid method, the H₃PO₄—CH₃COOH—Cu method and the H₃PO₄—HNO₃—CH₃COOH method are especially preferred.

With chemical polishing, a glossiness of at least 70% (in the case of rolled aluminum, at least 70% in both the rolling direction and the transverse direction) can be achieved.

Examples of electrolytic polishing methods include the various methods mentioned in the 6^(th) edition of Aluminum Handbook (Japan Aluminum Association, 2001), pp. 164-165.

A preferred example is the method described in U.S. Pat. No. 2,708,655.

The method described in Jitsumu Hyomen Gijutsu (Practice of Surface Technology), Vol. 33, No. 3, pp. 32-38 (1986) is also preferred.

With electrolytic polishing, a glossiness of at least 70% (in the case of rolled aluminum, at least 70% in both the rolling direction and the transverse direction) can be achieved.

Preferred use can also be made of the chemical-mechanical planarization (CMP) process employed in LSI chip fabrication.

These methods can be suitably combined and used. In a preferred example, a method that uses an abrasive is carried out by changing over time the abrasive used from coarser particles to finer particles, following which electrolytic polishing is carried out.

Mirror-like finishing enables a surface having, for example, a mean surface roughness R_(a) of 0.1 μm or less and a glossiness of at least 50% to be obtained. The mean surface roughness R_(a) is preferably 0.03 μm or less, and more preferably 0.02 μm or less. The glossiness is preferably at least 70%, and more preferably at least 80%.

The glossiness is the specular reflectance which can be determined in accordance with JIS Z8741-1997 (Method 3: 60° Specular Gloss) in a direction perpendicular to the rolling direction. Specifically, measurement is carried out using a variable-angle glossmeter (e.g., VG-1D, manufactured by Nippon Denshoku Industries Co., Ltd.) at an angle of incidence/reflection of 60° when the specular reflectance is 70% or less, and at an angle of incidence/reflection of 20° when the specular reflectance is more than 70%.

<Formation of Pits (Pre-Anodizing Treatment)>

Prior to anodizing treatment in which micropores are formed on the aluminum substrate (referred to below as the “main anodizing treatment”), it is preferable to form pits that will serve as the starting points for the formation of micropores in the main anodizing treatment.

No particular limitation is imposed on the method of forming pits. Examples of suitable methods include a self-ordering method which employs the self-ordering properties of the anodized layer, a physical method, a particle beam method, a block copolymer method, and a resist interference exposure method.

<Self-Ordering Method>

The self-ordering method is a method which enhances the orderliness by using the regularly arranging nature of micropores in the anodized layer and eliminating factors that disturb an orderly arrangement. Specifically, an anodized layer is formed on high-purity aluminum at a voltage appropriate for the type of electrolyte solution and at a low speed over an extended period of time (e.g., from several hours to well over ten hours), following which film removal treatment is carried out.

In this method, because the pore diameter is dependent on the voltage, the desired pore diameter can be obtained to a certain degree by controlling the voltage.

Typical examples of self-ordering methods include those described in J. Electrochem. Soc. Vol. 144, No. 5, p. L128 (May 1997); Jpn. J. Appl. Phys. Vol. 35, Part 2, No. 1B, p. L126 (1996); Appl. Phys. Lett. Vol. 71, No. 19, p. 2771 (Nov. 10, 1997), and in the above-referenced article by Masuda (1998). These self-ordering methods are carried out under the conditions indicated below.

-   (1) 0.3 mol/L sulfuric acid, 0° C., 27 V, 450 minutes (J.     Electrochem. Soc., 1997) -   (2) 0.3 mol/L sulfuric acid, 10° C., 25 V, 750 minutes (J.     Electrochem. Soc., 1997) -   (3) 0.3 mol/L oxalic acid, 17° C., 40 V, 600 minutes; followed by     pore widening treatment (solution containing 6 wt % phosphoric acid     and 1.8 wt % chromic acid, 60° C., 840 minutes) (Jpn. J. Appl.     Phys., 1996) -   (4) 0.3 mol/L oxalic acid, 17° C., 40 to 60 V, 36 minutes; followed     by pore widening treatment (5 wt % phosphoric acid, 30° C., 70     minutes) (Appl. Phys. Lett., 1997) -   (5) 0.04 mol/L oxalic acid, 3° C., 80 V, layer thickness, 3 μm;     followed by pore widening treatment (5 wt % phosphoric acid, 30° C.,     70 minutes) (Appl. Phys. Lett., 1997) -   (6) 0.3 mol/L phosphoric acid, 0° C., 195 V, 960 minutes; followed     by pore widening treatment (10 wt % phosphoric acid, 240 minutes)     (Masuda, 1998).

In the method described in these prior-art publications, film removal treatment to dissolve and remove the anodized layer is applied for at least 12 hours using a mixed aqueous solution of chromic acid and phosphoric acid at about 50° C. Carrying out treatment using a boiling aqueous solution destroys or disrupts the starting points for self-ordering. Hence, the aqueous solution is used without being boiled.

The regularity of the self-ordered anodized layer increases as the underlying aluminum is approached; once film removal has taken place, the lower portion of the anodized layer remaining on the underlying aluminum emerges at the surface, affording an orderly arrangement of pits. Therefore, in film removal treatment, only the anodized layer made of aluminum oxide is dissolved; the aluminum is not dissolved.

The self-ordering anodizing treatment used in this invention may be carried out by, for example, a method that involves passing an electrical current through the aluminum substrate as the anode in a solution having an acid concentration of 1 to 10 wt %. Solutions that may used in anodizing treatment include any one or combinations of two or more of the following: sulfuric acid, phosphoric acid, chromic acid, oxalic acid, sulfamic acid, benzenesulfonic acid and amidosulfonic acid.

The conditions of the self-ordering anodizing treatment vary empirically with the electrolyte solution used, although it is generally suitable for the electrolyte concentration to be 1 to 10 wt %, the temperature of the solution to be 0 to 20° C., the current density to be 0.1 to 10 A/dm², the voltage to be 10 to 200 V, and the period of electrolysis to be 2 to 20 hours.

The self-ordered anodized layer has a thickness of preferably 10 to 50 μm.

In the practice of the invention, the self-ordering anodizing treatment is carried out for a period of preferably 1 to 16 hours, more preferably 2 to 12 hours, and even more preferably 2 to 7 hours.

Film removal treatment is carried out for a period of preferably 0.5 to 10 hours, more preferably 2 to 10 hours, and even more preferably 4 to 10 hours.

After an anodized layer has been formed in this way by a self-ordering method, if the layer is then dissolved and removed (by film removal treatment), and the subsequently described main anodizing treatment is carried out under the same conditions, substantially straight micropores will be formed substantially perpendicularly to the surface of the layer.

<Physical Method>

Physical methods are exemplified by methods which use press patterning. A specific example is a method in which a substrate having on the surface a plurality of protrusions is pressed against the aluminum surface to form pits thereon. For instance, the method described in JP 10-121292 A can be used.

Another example is a method in which polystyrene spheres are densely arranged on the aluminum surface, SiO₂ is vapor-deposited onto the spheres and the aluminum surface, then the polystyrene spheres are removed and the substrate is etched using the vapor-deposited SiO₂ as the mask, thereby forming pits.

<Particle Beam Method>

In the particle beam method, pits are formed by irradiating the aluminum surface with a particle beam. This method has the advantage that the positions of the pits can be freely controlled.

Examples of the particle beam include a charged particle beam, a focused ion beam (FIB), and an electron beam.

An example of a particle beam method that can be used is the method described in JP 2001-105400 A.

<Block Copolymer Method>

The block copolymer method involves forming a block copolymer layer on the aluminum surface, forming a sea-island structure in the block copolymer layer by thermal annealing, then removing the island components to form pits.

An example of a block copolymer method that can be used is the method described in JP 2003-129288 A.

<Resist Interference Exposure Method>

In the resist interference exposure method, a resist is provided on the aluminum surface, then the resist is exposed and developed so as to form pits which pass entirely through the resist to the aluminum surface.

An example of a resist interference exposure method that can be used is the method described in JP 2000-315785 A.

Of the various above methods of forming pits, the self-ordering method, FIB method and resist interference exposure method are desirable because they are capable of uniformly forming pits over a large surface area of about 10 cm square or more.

From the standpoint of production costs, the self-ordering method is especially preferred. The FIB method is also desirable because it enables the arrangement of micropores to be controlled at will.

The pits formed have a depth of preferably at least about 10 nm and a width which is preferably not greater than the desired pore diameter.

<Main Anodizing Treatment>

As described above, it is preferable for an anodized layer having micropores therein to be formed by a main anodizing treatment step after pits have been formed on the aluminum surface.

The main anodizing treatment may be carried out under conditions known in the art, although the following conditions are preferred for obtaining an ordered arrangement.

The electrolyte solution has a concentration of preferably 0.01 to 1 mol/L, and more preferably 0.1 to 0.5 mol/L, and a temperature of preferably 0 to 20° C., and more preferably 0 to 18° C. The electrolysis voltage can be suitably selected according to the desired pore spacing and pore diameter, although it is preferable to select a voltage computed using formula (1) or (2) below. Voltage[V]=(basic period [nm]−13)/2.5 [nm/V]  (1) Voltage[V]=(desired pore diameter [nm]−12)/0.6 [nm/V]  (2)

In the above formulas, “basic period” refers to the center-to-center distance between neighboring micropores.

The above conditions are described in Masuda: Zairyo Gijutsu (Material Technology), Vol. 15, No. 10, p. 342 (1997).

When use is made of a process in which surface treatment is carried out after the anodized layer has been formed, according to one preferred embodiment, such treatment is carried out under the same conditions as the self-ordering method described above.

Suitable use can be made of a method in which the current is repeatedly turned on and off while maintaining a constant dc voltage, or a method in which the current is repeatedly turned on and off while intermittently varying the dc voltage. Micropores are formed in the anodized layer as a result of these methods.

In the above-described method in which the voltage is intermittently varied, it is preferable to successively lower the voltage. By doing so, the resistance of the anodized layer can be lowered.

When this main anodizing treatment is carried out at a low temperature, the arrangement of micropores is orderly and the pore diameter is uniform.

To facilitate the grain-forming treatment, it is preferable for the anodized layer to have a thickness which is from 0.5 to 10 times, preferably 1 to 8 times, and even more preferably 1 to 5 times, the pore diameter.

Therefore, according to one preferred embodiment, the anodized layer has a thickness of 0.1 to 1 μm and the micropores have a mean diameter of 0.01 to 0.5 μm.

<Dissolution Treatment>

Treatment in which the micropores are enlarged by dissolving and removing aluminum oxide that forms the anodized layer-is called “pore widening treatment.” Following the main anodizing treatment, pore widening treatment is carried out by immersing the aluminum substrate in an aqueous solution of an acid or an alkali so as to dissolve the anodized layer and enlarge the diameter of the micropores.

When pore widening treatment is to be carried out with an aqueous acid solution, it is preferable to use an aqueous solution of an inorganic acid such as sulfuric acid, phosphoric acid, nitric acid or hydrochloric acid, or a mixture thereof. It is desirable for the aqueous acid solution to have a concentration of 1 to 10 wt % and a temperature of 25 to 40° C.

When pore widening treatment is to be carried out with an aqueous alkali solution, it is preferable to use an aqueous solution of at least one alkali selected from the group consisting of sodium hydroxide, potassium hydroxide and lithium hydroxide. It is desirable for the aqueous alkali solution to have a concentration of preferably 0.1 to 5 wt % and a temperature of preferably 20 to 35° C.

Specific examples of preferred solutions include a 40° C. aqueous solution containing 50 g/L of phosphoric acid, a 30° C. aqueous solution containing 0.5 g/L of sodium hydroxide, and a 30° C. aqueous solution containing 0.5 g/L of potassium hydroxide.

The immersion time in the aqueous acid solution or aqueous alkali solution is preferably 8 to 60 minutes, more preferably 10 to 50 minutes, and even more preferably 15 to 30 minutes.

After pore widening treatment, the micropores have a mean pore diameter of preferably 10 to 500 nm, more preferably 15 to 450 nm, and even more preferably 20 to 400 nm; the coefficient of variation in the pore diameter is preferably 10 to 80%; and the mean pore density of the micropores is preferably from 50 to 1,500 pores/μm².

Anodizing treatment or dissolution treatment may be followed by hydrophilizing or hydrophobizing treatment for the subsequent treatments. The main purpose of hydrophilizing treatment is to impart wettability to the treatment liquids used in the various subsequent treatments (e.g., plating solution, electrolyte solution, coating fluid), and thus make it possible to achieve more uniform treatment in the subsequent treatments. Carrying out hydrophobizing treatment has the effect of preventing contamination from ambient sources such as solutions and the atmosphere.

No particular limitation is imposed on the hydrophilizing treatment method. Illustrative examples of suitable hydrophilizing treatments include the potassium hexafluorozirconate treatment described in U.S. Pat. No. 2,946,638, the phosphomolybdate treatment described in U.S. Pat. No. 3,201,247, the alkyl titanate treatment described in GB 1,108,559, the polyacrylic acid treatment described in DE 1,091,433 B, the polyvinylphosphonic acid treatments described in DE 1,134,093 B and GB 1,230,447, the phosphonic acid treatment described in JP 44-6409 B, the phytic acid treatment described in U.S. Pat. No. 3,307,951, the treatments involving the divalent metal salts of lipophilic organic polymeric compounds described in JP 58-16893 A and JP 58-18291 A, treatments like that described in U.S. Pat. No. 3,860,426 in which an aqueous metal salt (e.g., zinc acetate)-containing hydrophilic cellulose (e.g., carboxymethyl cellulose) layer is provided, and a treatment like that described in JP 59-101651 A in which a sulfo group-bearing water-soluble polymer is applied.

Additional examples of suitable hydrophilizing treatments include treatment involving application of the phosphates mentioned in JP 62-19494 A, the water-soluble epoxy compounds mentioned in JP 62-33692 A, the phosphoric acid-modified starches mentioned in JP 62-97892 A, the diamine compounds mentioned in JP 63-56498 A, the inorganic or organic salts of amino acids mentioned in JP 63-130391 A, the carboxyl or hydroxyl group-bearing organic phosphonic acids mentioned in JP 63-145092 A, the amino group and phosphonate group-containing compounds mentioned in JP 63-165183 A, the specific carboxylic acid derivatives mentioned in JP 2-316290 A, the phosphate esters mentioned in JP 3-215095 A, the compounds having one amino group and one phosphorus oxo acid group mentioned in JP 3-261592 A, the phosphate esters mentioned in JP 3-215095 A, the aliphatic or aromatic phosphonic acids (e.g., phenylphosphonic acid) mentioned in JP 5-246171 A, the sulfur atom-containing compounds (e.g., thiosalicylic acid) mentioned in JP 1-307745 A, and the phosphorus oxo acid group-bearing compounds mentioned in JP 4-282637 A.

It is preferable to carry out hydrophilizing treatment by a method in which the aluminum plate is immersed in an aqueous solution of an alkali metal silicate such as sodium silicate or potassium silicate, or by coating the plate with a hydrophilic vinyl polymer or a hydrophilic compound.

Hydrophilizing treatment with an aqueous solution of an alkali metal silicate such as sodium silicate or potassium silicate can be carried out according to the processes and procedures described in U.S. Pat. No. 2,714,066 and U.S. Pat. No. 3,181,461.

Illustrative examples of suitable alkali metal silicates include sodium silicate, potassium silicate and lithium silicate. The aqueous solution of an alkali metal silicate may include a suitable amount of, for example, sodium hydroxide, potassium hydroxide or lithium hydroxide.

An alkaline earth metal salt or a Group 4 (Group IVA) metal salt may also be included in the aqueous solution of an alkali metal silicate. Examples of suitable alkaline earth metal salts include nitrates such as calcium nitrate, strontium nitrate, magnesium nitrate and barium nitrate; and also sulfates, hydrochlorides, phosphates, acetates, oxalates, and borates. Exemplary Group 4 (Group IVA) metal salts include titanium tetrachloride, titanium trichloride, titanium potassium fluoride, titanium potassium oxalate, titanium sulfate, titanium tetraiodide, zirconyl chloride, zirconium oxide and zirconium tetrachloride. These alkaline earth metal salts and Group 4 (Group IVA) metal salts may be used singly or in combinations of two or more thereof.

Hydrophilizing treatment involving the formation of a hydrophilic layer can also be carried out in accordance with the conditions and procedures described in JP 59-101651 A and JP 60-149491 A.

Hydrophilic vinyl polymers that may be used in such a method include copolymers of a sulfo group-bearing vinyl polymerizable compound such as polyvinylsulfonic acid or sulfo group-bearing p-styrenesulfonic acid with a conventional vinyl polymerizable compound such as an alkyl (meth)acrylate. Examples of hydrophilic compounds that may be used in this method include compounds having at least one group selected from among —NH₂ groups, —COOH groups and sulfo groups.

Use can also be made of a method that involves applying a liquid containing the above-described hydrophilizing agent to the surface of the anodized layer and drying.

No particular limitation is imposed on the hydrophobizing treatment. Illustrative examples of suitable hydrophobizing treatments include methods that involve the formation of a hydrophobic layer using carboxymethyl cellulose; dextrin; gum arabic; amino group-bearing phosphonic acids such as 2-aminoethylphosphonic acid, and organophosphonic acids which may have substituents, such as phenylphosphonic acid, naphthylphosphonic acid, alkylphosphonic acids, glycerophosphonic acids, methylene diphosphonic acid and ethylene diphosphonic acid; organophosphoric acids which may have substituents, such as phenylphosphoric acid, naphthylphosphoric acid, alkylphosphoric acid and glycerophosphoric acid; organophosphinic acids which may have substituents, such as phenylphosphinic acid, naphthylphosphinic acid, alkylphosphinic acid and glycerophosphinic acid; amino acids such as glycine and β-alanine; and the hydrochlorides of hydroxyl group-bearing amines, such as the hydrochloride of triethanolamine. These may be used singly or as combinations of two or more thereof.

The layer containing a polymeric compound having an acid group-bearing component and an onium group-bearing component described in JP 2000-105462 A can also be advantageously used as the hydrophobic layer.

Use can also be made of a method that involves applying a liquid containing the above-described hydrophobizing agent to the surface of the anodized layer and drying.

No particular limitation is imposed on the method of application that may be used in hydrophilizing treatment and hydrophobizing treatment. Suitable examples of such methods include bar coating, spin coating, spray coating, curtain coating, dip coating, air knife coating, blade coating and roll coating.

<Sealing Treatment>

The microstructure of the invention may additionally be subjected to sealing treatment in which the micropores in the anodized layer are filled with metal.

In the practice of the invention, the metal is not subject to any particular limitation, so long as it is an element having metal bonds that include free electrons. However, a metal in which plasmon resonance has been recognized is preferred. Of these, it is known that gold, silver, copper, nickel and platinum are known to readily give rise to plasmon resonance (Gendai Kagaku (Contemporary Chemistry), pp. 20-27 (September 2003)), and are thus preferred. Gold and silver are especially preferred because of the ease of electrodeposition and colloidal particle formation.

Sealing may be carried out using any suitable known technique without particular limitation. Examples of preferred techniques include electrodeposition, and a method which involves coating the microstructure having an anodized layer with a dispersion of metal colloidal particles, then drying. The metal is preferably in the form of single particles or agglomerates.

An electrodeposition method known in the art may be used. For example, in the case of gold electrodeposition, use may be made of a process in which the microstructure is immersed in a 30° C. dispersion containing 1 g/L of HAuCl₄ and 7 g/L of H₂SO₄ and electrodeposition is carried out at a constant voltage of 11 V (regulated with a variable autotransformer) for 5 to 6 minutes.

An example of an electrodeposition method which employs copper, tin and nickel is described in detail in Gendai Kagaku (Contemporary Chemistry), pp. 51-54 (January 1997)). Use can be made of this method as well.

The dispersions employed in sealing methods which use metal colloidal particles can be obtained by a known method. Illustrative examples include methods of preparing fine particles by low-vacuum vapor deposition and methods of preparing metal colloids by reducing an aqueous solution of a metal salt.

The metal colloidal particles have a mean particle size of preferably 1 to 200 nm, more preferably 1 to 100 nm, and even more preferably 2 to 80 nm.

Preferred use can be made of water as the dispersion medium employed in the dispersion. Use can also be made of a mixed solvent composed of water and a solvent that is miscible with water, such as an alcohol, illustrative examples of which include ethyl alcohol, n-propyl alcohol, i-propyl alcohol, 1-butyl alcohol, 2-butyl alcohol, t-butyl alcohol, methyl cellosolve and butyl cellosolve.

No particular limitation is imposed on the technique used for coating the aluminum substrate having an anodized layer with the dispersion of metal colloidal particles. Suitable examples of such techniques include bar coating, spin coating, spray coating, curtain coating, dip coating, air knife coating, blade coating and roll coating.

Preferred examples of dispersions that may be employed in sealing methods which use metal colloidal particles include dispersions of gold colloidal particles and dispersions of silver colloidal particles.

Dispersions of gold colloidal particles that may be used include those described in JP 2001-89140 A and JP 11-80647 A. Use can also be made of commercial products.

Dispersions of silver colloidal particles preferably contain particles of silver-palladium alloys because these are not affected by the acids which leach out of the anodized layer. The palladium content in such a case is preferably from 5 to 30 wt %.

After the anodized aluminum plate has been coated with the dispersion, it may be suitably cleaned using a solvent such as water. As a result of such cleaning, only the particles filled into the micropores remain on the anodized layer; particles that have not been filled into micropores are removed.

The amount of metal deposited after sealing is preferably 100 to 500 mg/m².

The surface porosity following sealing treatment is preferably not more than 20%. The surface porosity following sealing treatment is defined as the total surface area of the openings in unsealed micropores relative to the surface area of the aluminum surface. When the surface porosity is in the above range, a stronger localized plasmon resonance can be obtained.

At a pore diameter of 50 nm or more, it is preferable to use a sealing method that employs metal colloidal particles. At a pore diameter of less than 50 nm, the use of an electrodeposition process is preferred. Suitable use can also be made of a combination of both.

<Microstructure>

The inventive microstructure obtained as described above has a large mean domain size on the surface of the anodized layer and is highly ordered. The microstructure of the invention can be used, inter alia, in the field of biosensing, such as a Raman spectroscopy sample holder having a structure in which the micropores on the anodized layer are filled with metal.

In particular, when the inventive microstructure is used in applications that utilize its optical transparency, as the region of high regularity in the pore arrangement becomes larger, the sensing light reaches the detector without undergoing irregular reflection, resulting in a high measurement precision. Moreover, if the micropores in the inventive microstructure are sealed with gold, there can be obtained a microstructure in which the micropore interiors have been filled with gold in an orderly manner and which is thus useful as a sensing device.

EXAMPLES

Examples are given below by way of illustration and should not be construed as limiting the invention.

1. Fabrication of Microstructure

(1) Aluminum Plate

Melts were prepared from aluminum materials having the compositions shown in Table 1 below, with the balance being aluminum and inadvertent impurities. The melts were subjected to molten metal treatment and filtration, then were cast into 500 mm thick, 1,200 mm wide ingots by a direct chill casting process. The ingots were scalped with a scalping machine, removing an average of 10 mm of material from the surface, then soaked and held at 550° C. for about 5 hours. When the temperature had fallen to 400° C., the ingots were rolled with a hot rolling mill to a thickness of 2.7 mm. In addition, heat treatment was carried out at 500° C. in a continuous annealing furnace, following which cold rolling was carried out to a final thickness of 0.24 mm and a plate width of 1030 mm. Heat treatment was then carried out under the conditions indicated in Table 2 (temperature, treatment time, and number of heat treatments), following which mirror-like finishing, pre-anodizing treatment (pit formation), film removal treatment and main anodizing treatment were carried out in this order to give finished microstructures. In Table 2, a dash (-) indicates that the treatment in question was not carried out. Table 1 gives the results obtained from measurement of the Vickers hardness without heat treatment of the aluminum material prior to mirror-like finishing and other subsequent operations, and from measurement of the Vickers hardness after 2 hours of heat treatment at 300° C.

Measurement of Vickers Hardness

In accordance with JIS Z2251, test specimens (10 mm square flat plates) were prepared and a dent (indentation) was made on the surface of each of the above-described aluminum materials using a square-based diamond indenter having an angle of 136° between the opposite faces under a test load of 200 gf. The size of the diagonals in the identation was measured from a scanning electron micrograph, and the Vickers hardness was computed according to the formula provided in JIS Z2251. TABLE 1 Vickers hardness Aluminum Al composition (wt %) Not heat Heat material Si Fe Cu Ti treated treated Aluminum 0.0013 0.0015 0.0070 0.0001 38 19 material 1 Aluminum 0.0087 0.0015 0.0080 0.0002 41 19 material 2 Aluminum 0.0130 0.0018 0.0070 0.0002 50 28 material 3

Each type of treatment used in the examples is described below.

(2) Mirror-Like Finishing

Polishing with an abrasive cloth, buffing, then electrolytic polishing were carried out in this order for mirror-like finishing. After buffing, the plate was rinsed with water.

Polishing with an abrasive cloth was carried out using a polishing platen (Abramin, produced by Marumoto Struers K.K.) and commercial water-resistant abrasive cloths. This polishing operation was carried out while successively changing the grit of the water-resistant abrasive cloths in the following order: #200, #500, #800, #1000 and #1500.

Buffing was carried out using slurry-type abrasives (FM No. 3 (mean particle size, 1 μm) and FM No. 4 (mean particle size, 0.3 μm), both made by Fujimi Incorporated).

Electrolytic polishing was carried out for 2 minutes using an electrolyte solution of the composition indicated below (temperature, 70° C.), using the substrate as the anode and a carbon electrode as the cathode, and at a constant current of 130 mA/cm². The power supply was a GP0110-30R unit manufactured by Takasago, Ltd. <Electrolyte Solution Composition> 85 wt % Phosphoric acid (Wako Pure Chemical 660 mL Industries, Ltd.) Pure water 160 mL Sulfuric acid 150 mL Ethylene glycol  30 mL (3) Formation of Pits

The self-ordering method described below was used to form, in the surface of the mirror-like finished aluminum plate, pits intended to serve as the starting points of micropore formation in the subsequently described main anodizing treatment.

<Self-Ordering Method (Pre-Anodization)>

The aluminum plate was immersed in an aqueous solution containing 0.5 mol/L of oxalic acid at 16° C., and the surface was subjected to self-ordering anodization by carrying out 5 hours of low-voltage electrolytic treatment at a voltage of 40 V and a current density of 1.4 A/dm². In self-ordering anodization, use was made of NeoCool BD36 (Yamato Scientific Co., Ltd.) as the cooling system, Pairstirrer PS-100 (Tokyo Rikakikai Co., Ltd.) as the stirring and warming unit, and a GP0650-2R unit (Takasago, Ltd.) as the power supply.

<Film Removal Treatment>

Next, the substrate on which an anodized layer had been formed was immersed for 12 hours in a treatment solution (solution temperature, 50° C.) composed of 118 g of an aqueous solution containing 85 wt % of phosphoric acid (available from Kanto Chemical Co., Inc.), 30 g of anhydrous chromic acid (Kanto Chemical Co., Inc.) and 1500 g of pure water, thereby carrying out film removal treatment in which the anodized layer was dissolved.

The thickness of the anodized layer after film removal treatment was 0.1 μm or less.

(4) Main Anodizing Treatment A

The main anodizing treatment was carried out on the substrate having pits formed thereon. This step was carried out by immersing the substrate at 16° C. in an aqueous solution containing 0.5 mol/L of oxalic acid, and subjecting the immersed substrate to low-voltage electrolytic treatment at a voltage of 40 V and a current density of 1.4 A/dm² for 2 minutes.

Measurement of Domain Size

FIG. 2 is a schematic diagram illustrating the method for measuring domain size. Regularly arranged micropores are represented by open circles. When this array of micropores is connected by straight lines, as shown in FIGS. 1 and 2, there arise places where shifts in the array occur. These places where shifting has occurred were treated as the boundaries, and the regions enclosed by such boundaries were regarded as domains. Given the actual domain size and the size of the micropores, some 20 to 30 micropores created by ordering anodization with oxalic acid are arrayed in the respective directions (L1, L2 and L3 in FIG. 2) measured in each domain. To measure the domain size, the micropores on the aluminum surface following main anodizing treatment were photographed at a tilt angle of 0° and a magnification of 20,000 using a field emission-type scanning electron microscope (FE-SEM). In the resulting image, the pore array was connected by straight lines in directions L1, L2 and L3 that intersect at 60° intervals as shown in FIG. 2, the length over which the array continues without disruption was measured in each direction, and the average of those lengths was expressed as a single domain size. Images were taken over a field corresponding to approximately 1×10⁻³ mm², the size was measured for at least ten or more domains, and the domain sizes were averaged to give a mean domain size. The starting point of domain size measurement shown in FIG. 2 was obtained by choosing several pores located at approximately the center of a domain, and selecting that point which maximizes the size of the domain. The results are shown in FIG. 2 TABLE 2 Number of heat treatments 0 1 1 1 1 1 1 2 2 Heat treatment temp. (° C.) 0 150 200 250 300 350 400 250 300 Heat 0 38 — — — — — — — — treatment 0.5 — 38 38 36 33 33 31 — — time (<2 μm) (<2 μm) (<2 μm) (<2 μm) (<2 μm) (<2 μm) (hours) 1 — 37 29 28 26 22 20 — — (<2 μm) (≧2 μm) (≧2 μm) (≧3 μm) (≧3 μm) (≧3 μm) 2 — 35 27 23 19 18 18 20 16 (<2 μm) (≧2 μm) (≧3 μm) (≧5 μm) (≧5 μm) (≧5 μm) (≧2 μm) (≧5 μm) 3 — 32 26 21 18 18 18 — — (<2 μm) (≧3 μm) (≧3 μm) (≧5 μm) (≧5 μm) (≧5 μm) Note: The measured values indicated in the table are Vickers hardness. The values in parentheses are the results of mean domain size (μm) measurements. The aluminum material was obtained by subjecting Aluminum Material 1 in Table 1 to anodizing treatment with an electrolyte solution of oxalic acid. <2 μm ≧2 μm: at least 2 μm but less than 3 μm ≧3 μm: at least 3 μm but less than 5 μm ≧5 μm

As is apparent from Table 2, microstructures obtained by anodizing an aluminum plate with a Vickers hardness of up to 20 have a mean domain size of at least 2 μm, and thus excellent regularity.

2. Fabrication of Microstructure

Aside from using main anodizing treatment B (sulfuric acid electrolyte solution) described below as the main anodizing treatment, microstructures were similarly obtained by using the same Aluminum Materials 1 to 3 and carrying out heat treatment in the same way as indicated in Table 2. As when main anodizing treatment A (oxalic acid electrolyte solution) was carried out, the resulting microstructures retained a mean domain size of 2 μm or more.

(4)′ Main Anodizing Treatment B

The substrate on which pits had been formed was immersed in an aqueous solution containing 0.3 mol/L of sulfuric acid at 16° C. and subjected to low-voltage electrolytic treatment at a voltage of 40 V and a current density of 1.4 A/dm² for 2 minutes, thus carrying out main anodizing treatment B.

3. Fabrication of Microstructure With/Without Hydrophilizing Treatment

Three types of microstructures that had been subjected to, of the heat treatment conditions indicated in Table 2 above, two hours of heat treatment at 150° C., 250° C. or 350° C. were subjected to the same type of treatment as described in section 1. above and anodizing treatment, after which some specimens of each type of microstructure were subjected to hydrophilizing treatment, thereby giving both microstructures that had been hydrophilized and microstructures that had not been hydrophilized. The microstructures thus obtained were evaluated. The results are shown in Table 3.

(5) Hydrophilizing Treatment

Treatment was carried out by 10 seconds of immersion in an aqueous solution containing 1 wt % of sodium silicate at 35° C.

(6) Measurement of Contact Angle

Each of the microstructures was immersed in a cell filled with Swasol (a petroleum-based hydrocarbon mixture), and the contact angle of a water droplet on the microstructure surface in Swasol was measured. Measurement was carried out using a Face contact angle meter (model CA-X manufactured by Kyowa Interface Science Co., Ltd.). The results are presented in Table 3. Each value shown is the average of five measurements.

(7) Evaluation of Gold Filling Uniformity

Filling treatment (gold electrodeposition) was carried out on the microstructures as follows. Each microstructure was immersed in a 30° C. dispersion containing 1 g/L of HAuCl₄ and 7 g/L of H₂SO₄, and electrodeposition treatment was carried out at a constant voltage of 11 V (regulated with a variable autotransformer) for 5 to 6 minutes. The uniformity of filling in the resulting gold-electrodeposited microstructures was visually evaluated by examining FE-SEM images obtained in the same way as for domain size evaluation. The results indicated that the uniformity improves at lower Vickers hardness, and improves even further with hydrophilizing treatment.

Very Good (VG): Size of filled gold (Au) (i.e., shape as seen from above in an FE-SEM image) is uniform.

Good: Size is uniform at least within domains

Pass: Size is not uniform even within domains, but no areas are unfilled.

Fail: Some areas are unfilled.

The results are shown in Table 3. TABLE 3 Heat Gold filling uniformity/Contact angle treatment Vickers No Hydrophilizing temperature hardness hydrophilizing treatment (° C.) (Hv) treatment carried out 150 35 Fail/48° Pass/13° 250 23 Good/50° VG/15° 350 18 VG/42° VG/12° 

1. A microstructure comprising regularly arranged micropores which is obtained by anodizing an aluminum layer having a Vickers hardness Hv of up to
 20. 2. A method of producing a microstructure comprising the steps of: heat-treating an aluminum layer at least once at a temperature of at least 200° C. for at least one hour; and anodizing the heat-treated aluminum layer.
 3. The microstructure of claim 1, wherein the regularity of the micropores, expressed as the mean domain size, is at least 2 μm.
 4. The method of claim 2, wherein the regularity of the micropores, expressed as the mean domain size, is at least 2 μm. 